1. Field of the Invention
This disclosure relates to a real-time, hybrid amplitude-time division polarimetric imaging camera used to derive and calculate Stokes parameters of input light.
2. Description of the Related Art
Traditional imaging systems have long attempted to produce an image capable of revealing useful details to the viewer. In low light-level scenes, for example, imaging can be particularly problematic since traditional cameras do not produce the image-clarity and contrast required to reveal details shrouded in the darker recesses of the image. In applications such as military operations, rescue missions or law enforcement work, for example, object differentiation and clarity is critical.
Some recent approaches to imaging systems have utilized light polarization techniques to process images. A common polarimetric image processing technique involves processing four different linear polarization states, each offset from the other by 45 degrees. Typically, these four polarization states are sufficient to detect the polarization signature caused by light reflection from the surface of an object, which can be useful for differentiating man-made objects from the natural background.
In a well-known polarimetric imaging technique called Time Division Method (TDM) (also known as Time Sequential Method), each one of the four polarization states of an image is captured sequentially in time. This requires rotating the polarizer and capturing the corresponding image four times to obtain the complete set of polarimetric image parameters. One major problem with this approach is motion-induced polarization artifacts that are produced with slight movement in the object/scenery between each measurement when the images shift by even a fraction of a pixel. This problem is especially pronounced in common manually controlled polarization rotators, whose rotation time is in the order of seconds. However, the artifact problem also arises in electrically controlled polarization rotators, such as a liquid crystal polarization rotator. Despite their electrical operation, these polarization rotators lack the speed required to minimize the motion-induced polarization artifacts.
Another commonly used approach for obtaining the four polarization state parameters of a scene is the Aperture Division Method (“ApDM”). In ApDM, an array of micropolarizers arranged in front of an imaging sensor measures and maps the four polarization state parameters to a single polarimetric image pixel. Each polarimetric image pixel comprises a group of four image sensor pixels (typically 2 by 2), and in front of each image sensor pixel is a micropolarizer with its polarization axis aligned with one of the four polarization states. The four imaging sensor pixels detect the four polarization states simultaneously to produce one polarimetric image pixel. The ApDM does not have the motion-induced false polarization artifact associated with the TDM. The use of micropolarizers poses several problems, however. One problem is the loss of resolution due to the use of four sensor elements to form a single image pixel. Another problem is the spatial variance in the four pixels that arises because the light falling on the four sensor pixels is not actually from the same object point. This requires ApDM to employ a post image processing technique to merge the four pixels using software correction, which can add complexity and additional processing power requirements to the system. Another problem with this technique is the crosstalk and limited polarization extinction ratio caused by the light diffracting from a polarizer of one sensor pixel onto another sensor pixel. This problem typically arises because the micropolarizers are made of lithographically written wiregrids and the diffraction from these devices can be significant. Another problem is the requirement of pixel matching between the micropolarizer array and the imaging sensor. For each kind of imaging sensor (or focal plane array), a unique array of polarizers must be developed and fabricated, and the sensor and polarizer array must be aligned and bonded with very high precision. This increases costs over using commercially available imaging sensors.
A third technique for detecting polarimetric images is the Amplitude Division Method (“AmDM”). The AmDM involves using a beam splitter to divide the input light into four images, which are then each polarized and processed separately. Problems with this approach include additional light loss, image aberration management in each arm, and pixel matching of several imaging sensors.
These techniques do not provide the desired polarimetric images having high polarization extinction, absence of motion-induced false polarization signature, small form factor, and high image resolution.